Heretofore, a hot-dip galvanized steel sheet has been used mainly in the automotive field. However, in the case of a hot-dip galvanized steel sheet using, as the substrate, a high-strength (for example, a tensile strength of 980 MPa or more) steel sheet, a crack may readily occur particularly in the underlying steel sheet during severe working thereof such as bending, and a stress concentration on that portion may cause a fracture in many cases.
From this standpoint, for example, Patent Document 1 has proposed to reduce the amount of Si enrichment in the surface of a steel sheet before the immersion thereof in a plating bath, to a certain value or less, by controlling an annealing atmosphere. However, the control itself to such an annealing atmosphere has been difficult.
Also, Patent Document 2 describes a high-strength steel sheet where Si+Al satisfies 0.7% or more and where as the steel sheet structure, the area ratio of the total amount of lower bainite and total martensite to the whole steel sheet structure is from 10 to 90%, the amount of retained (or residual) austenite is from 5 to 50%, and the area ratio of bainitic ferrite in upper bainite to the whole steel sheet structure is 5% or more. Patent Document 3 describes an alloyed hot-dip galvanized steel sheet having a microstructure containing, in terms of area ratio, from 20 to 75% of ferrite and from 5 to 25% of retained austenite, where the average crystal grain size of the ferrite is 10 μm or less. Patent Document 4 describes an alloyed hot-dip galvanized steel sheet having formed in the surface thereof, starting from the steel sheet side, a Γ phase, a mixed layer of Γ1 phase and δ1 phase, and a δ1 phase, or a δ1 phase and a ζ phase, wherein the average thickness of Γ phase is 1.5 μm or less, the average thickness of the mixed layer of Γ1 phase and δ1 phase is less than two times the average thickness of Γ phase, and the average aspect ratio (ratio of long side to short side in cross-sectional observation) of Γ1 crystal is 2 or more.
Further, Patent Document 5 describes an alloyed hot-dip galvanized steel sheet where the alloyed hot-dip galvanized layer has a chemical composition containing, in mass %, Fe: from 10 to 15% and Al: from 0.20 to 0.45, with the balance being Zn and impurities, and the interface adhesion strength between the steel sheet and the alloyed hot-dip galvanized layer is 20 MPa or more. Patent Document 6 describes an alloyed hot-dip galvanized steel sheet excellent in impact resistance and adhesion, having a coating weight of 20 to 100 g/m2 on one surface or both surfaces, wherein the average Fe content of the plating layer is from 8 to 16% and the thickness of Γ phase in the plating layer is from 0.2 to 1.5 μm. Patent Document 7 describes a hot-dip galvanized steel sheet having a galvanized film in which an Fe—Al-based alloy layer, an Fe—Zn-based alloy layer and a zinc plating layer are present in this order starting from the base steel sheet side, wherein the Al content in the Fe—Al-based alloy layer is from 10 to 300 mg/m2 and the thickness of the Fe—Zn-based alloy layer is ½ or less of the thickness of the galvanized film. Patent Document 8 describes an alloyed hot-dip galvanized steel sheet, wherein the number of iron-zinc alloy crystals in contact with the plating film/base iron interface is 5.5 or more per 1 μm of the interface.
However, with respect to a high-strength steel sheet having a high strength (for example, a tensile strength of 980 MPa or more), a hot-dip galvanized steel sheet exhibiting a sufficient effect may not be known.